The present invention relates generally to microlasers and associated fabrication methods and, more particularly, to side pumped Q-switched microlasers and associated fabrication methods.
Modern electro-optical applications are demanding relatively inexpensive, miniaturized lasers capable of producing a series of well-defined output pulses. As such, a variety of microlasers have been developed which include a microresonator and a pair of at least partially reflective mirrors disposed at opposite ends of the microresonator to define a resonant cavity therebetween. The microresonator of one advantageous microlaser includes an active gain medium and a saturable absorber that serves as a Q-switch. See, for example, U.S. Pat. No. 5,394,413 to John J. Zayhowski which issued on Feb. 28, 1995, the contents of which are incorporated in their entirety herein. By appropriately pumping the active gain medium, such as with a laser diode, the microresonator will emit a series of pulses having a predetermined wavelength, pulse width and pulse energy.
As known to those skilled in the art, the wavelength of the signals emitted by a microlaser is dependent upon the materials from which the active gain medium and the saturable absorber are formed. In contrast, the pulse width of the laser pulses emitted by a conventional microlaser is proportional to the length of the resonator cavity. As such, longer resonator cavities will generally emit output pulses having greater pulse widths. Further, both the pulse energy and average power provided by a microlaser are proportional to the pulse width of the pulses output by the microlaser. All other factors being equal, the longer the microresonator cavity, the longer the pulse width and the greater the pulse energy and average power of the resulting laser pulses.
Conventional microlasers, such as those described by U.S. Pat. No. 5,394,413, are end pumped in a direction parallel to the longitudinal axis defined by the resonator cavity. In this regard, the longitudinal axis of the microresonator cavity extends lengthwise through the resonator cavity and is oriented so as to be orthogonal to the pair of at least partially reflective mirrors that define the opposed ends of the resonant cavity. As such, conventional microlasers are configured such that the pump source provides pump signals in a direction perpendicular to the at least partially reflective mirrors that define the opposed ends of the resonant cavity. The effective length of the resonator cavity is therefore equal to the physical length of the resonator cavity.
While the microlaser can be fabricated such that the resonator cavity has different lengths, a number of factors contribute to generally limit the permissible length of the resonator cavity. See, for example, U.S. Pat. No. 5,394,413 that states that the resonator cavity, including both the saturable absorber and the gain medium, is preferably less than two millimeters in length. In particular, a number of electro-optical applications require microlasers that are extremely small. As such, increases in the length of the resonator cavity are strongly discouraged in these applications since any such increases in the length of the microresonator cavity would correspondingly increase the overall size of the microlaser.
In addition, the length of passively Q-switch microlasers is effectively limited by the requirement that the inversion density must exceed a predetermined threshold before lasing commences. As the physical length of the resonator cavity increases, greater amounts of pump energy are required in order to create the necessary inversion density for lasing. In addition to disadvantageously consuming more power to pump the microlaser, the increased pumping requirements create a number of other problems, such as the creation of substantially more heat within the microlaser which must be properly disposed of in order to permit continued operation of the microlaser. In certain instances, the heat generated within the microlaser may even exceed the thermal capacity of the heat sink or other heat removal device, thereby potentially causing a catastrophic failure of the microlaser.
Since the pulse width and correspondingly the pulse energy and average power of the pulses output by a microlaser cavity are proportional to the length of the resonator cavity, the foregoing examples of practical limitations on the length of the resonator cavity also disadvantageously limit the pulse width and the corresponding pulse energy and average power of the pulses output by the conventional microlasers. However, some modern electro-optical applications are beginning to require microlasers that emit pulses having greater pulse widths, such as pulse widths of greater than 1 nanosecond and, in some instances, up to 10 nanoseconds, as well as pulses that have greater pulse energy, such as between about 10 xcexcJ and about 100 xcexcJ, and greater average power, such as between 0.1 watts and 1 watt. As a result of the foregoing limitations on the length of the resonator cavity and the corresponding limitations on the pulse widths, pulse energy and average power of the pulses output by the conventional microlasers, conventional microlasers do not appear capable of meeting these increased demands.
A microlaser is therefore provided according to one embodiment of the present invention that is capable of supporting a zig-zag resonation pattern in response to side pumping of the active gain medium so as to effectively lengthen the microresonator cavity without having to physically lengthen the microresonator cavity. As such, the microlaser of this embodiment can generate pulses having greater pulse widths and correspondingly greater pulse energies and average power levels than the pulses provided by conventional microlasers of a similar size. A corresponding fabrication method is also provided according to one embodiment of the present invention that permits a plurality of side pumped Q-switched microlasers to be fabricated in an efficient and repeatable manner.
According to the present invention, the microlaser includes a microresonator having an active gain medium and a Q-switch, such as a passive Q-switch. The microresonator extends lengthwise between opposed end faces and has a first side surface extending between the opposed end faces. The microlaser also includes first and second reflective surfaces disposed proximate respective ones of the opposed end faces to define a microresonator cavity therebetween. While the first and second reflective surfaces can be coated upon respective ones of the opposed end faces of the microresonators, the first and second reflective surfaces can also be formed by mirrors that are spaced from respective ones of the opposed end faces The microlaser can also include a pump source for introducing pump signals into the active gain medium via the first side surface of the microresonator such that the zig-zag resonation pattern is established within the microresonator cavity.
In one advantageous embodiment, the opposed end faces are each disposed at a nonorthogonal angle ∀, such as between about 300 and about 350, relative to a line perpendicular to a longitudinal axis defined by the microresonator cavity and extending between the opposed end faces. In one embodiment, the opposed end faces are each disposed at the same nonorthogonal angle ∀ relative to the longitudinal axis such that the opposed end faces are parallel. In another embodiment, the opposed end faces are oriented in opposite directions by the same nonorthogonal angle ∀ relative to the longitudinal axis. As a result of the nonorthogonal relationship of the opposed end faces to the longitudinal axis defined by the microresonator cavity, the microlaser of either embodiment is capable of supporting the zig-zag resonation pattern in response to side pumping of the active gain medium via the first side surface of the microresonator.
By supporting the zig-zag resonation pattern, the effective length of the microresonator cavity is increased relative to conventional microlasers having substantially the same physical size. In this regard, the effective length of the microresonator cavity of the present invention is the length of the zig-zag resonation path established by the microlaser which is significantly longer than the linear resonation paths established by conventional microlasers that extend parallel to the longitudinal axis of the resonator cavity. As such, the microlaser of the present invention can emit pulses having a longer pulse width and correspondingly greater pulse energies and average power levels than the pulses emitted by conventional microlasers of the same physical size.
In order to permit the pump signals to be received by the active gain medium without being reflected from the first side surface, the microlaser can include an antireflection coating on the first side surface for permitting pump signals having a predetermined range of wavelengths to be received by the active gain medium. In addition to the first side surface, the microresonator generally includes a second side surface opposite the first side surface and extending between the opposed end faces. The microlaser of this embodiment can further include a reflectance coating upon the second side surface for reflecting the pump signals, thereby insuring that the pump signals that have entered the active gain medium remain within the active gain medium. In one embodiment, the microresonator also includes third and fourth opposed side surfaces extending between the opposed end faces and between the first and second side surfaces. In order to further facilitate resonation within the microresonator cavity, the third and fourth side surfaces can be roughened, such as by grinding, to thereby diffuse light.
In order to permit the microlaser to emit signals of a predetermined lasing wavelength via one of the opposed end faces, the first reflective surface is preferably highly reflective for laser signals having the predetermined lasing wavelength. In contrast, the second reflective surface is preferably only partially reflective for laser signals having the predetermined lasing wavelength. As such, the microlaser can emit laser pulses having the predetermined lasing wavelength via the second reflective surface.
In one embodiment, the microlaser also includes a heat sink upon which the microresonator is mounted and a housing in which the microresonator and the pump source are so disposed. In this embodiment, the housing includes a window through which laser signals generated by the microresonator are emitted. In order to protect the microresonator, such as from deleterious environmental conditions, the microlaser can also include another window disposed within the housing for separating the pump source from the microresonator such that the microresonator is disposed in a portion of the housing that can be sealed.
According to another embodiment of the present invention, a method of fabricating a plurality of side pumped, passively Q-switched microlasers is provided. This method initially provides a layer of passive Q-switch material. Thereafter, the active gain medium is grown, such as by liquid phase epitaxy, upon the layer of passive Q-switch material to form a composite structure having opposed major surfaces. While the active gain medium and passive Q-switch material can be formed of a variety of materials, the method of one advantageous embodiment grows neodymium-doped yttrium aluminum garnet (YAG) upon a layer of tetravalent chrome-doped YAG that serves as the passive Q-switch material.
The resulting composite structure is then cut at a nonorthogonal angle xcex1 relative to the opposed major surfaces to thereby form the plurality of passively Q-switched microlasers. By cutting at a nonorthogonal angle xcex1 relative to the opposed major surfaces, each passively Q-switched microlaser defines a longitudinal axis and has opposed end faces that are disposed at the same nonorthogonal angle xcex1 with respect to the longitudinal axis. As such, a plurality of side pumped passively Q-switched microlasers can be fabricated according to this embodiment of the present invention in an efficient and repeatable manner.
In one advantageous embodiment, the composite structure is divided into a plurality of bars prior to cutting the composite structure at the nonorthogonal angle xcex1. In this embodiment, each respective bar is thereafter cut at the nonorthogonal angle xcex1 relative to the opposed major surfaces to form the plurality of passively Q-switched microlasers. After cutting the composite structure at the nonorthogonal angle xcex1, the first side surface of each microlaser can be coated with the antireflection coating to permit pump signals having a predetermined range of wavelengths to be received by the active gain medium without being reflected from the first side surface. In addition, the second side surface of each microlaser, opposite the first side surface, can be coated with a reflectance coating for internally reflecting the pump signals. Moreover, the third and fourth opposed side surfaces of each microlaser can be roughened, such as by finely grinding, to diffuse light.
Additionally, the opposed ends of the microlaser can be coated with first and second reflective surfaces, typically after cutting the composite structure at the nonorthogonal angle xcex1. In this regard, the first reflective surface is highly reflective for signals having the predetermined lasing wavelength, while the second reflective surface is only partially reflective for signals having the predetermined lasing wavelength. As such, the resulting microlasers will advantageously be capable of supporting a zig-zag resonation pattern and of controllably emitting pulses of the predetermined lasing wavelength via the second reflective surface.